1. Field of the Invention
The present invention relates to an image forming apparatus including a fixing device that heats and fixes a toner image transferred onto a sheet.
2. Description of the Related Art
Conventionally, an electrophotographic scheme has been widely adopted in image forming apparatuses, such as copiers and printers. An image forming apparatus adopting the electrophotographic scheme develops an electrostatic latent image formed on an image carrier with toner to form a visual image (form a toner image), transfers the image onto a sheet and subsequently fixes the image with heat and pressure. Thus, the image forming apparatus adopting the electrophotographic scheme includes a fixing device that fixes an unfixed toner image on the sheet by heating and applying pressure.
The fixing device typically includes a fixing roller or a fixing film as a heater, and a pressure roller pressed against the heater, and conveys a sheet while sandwiching the sheet at a nip between the heater and the pressure roller, thereby heating and applying pressure to an unfixed toner image transferred onto the sheet. Accordingly, when the sheet passes the nip between the heater and the pressure roller, moisture on the surface on one side of the sheet (surface on a heated side) may sometimes be evaporated by heating to make opposite ends of the sheet in the conveying direction (front and rear ends) curl.
Although depending on the material and the fibrous direction of the sheet in addition to the amount of moisture in the sheet, the sheet typically curls such that a surface where the amount of moisture is reduced by evaporation of the moisture due to heating shrinks. The sheet thus curls on a side with a relatively large shrinkage. Curling of the opposite ends in the sheet conveying direction causes a possibility of a winding jam where a sheet winds around a conveyor roller conveying the sheet. The winding jam is not preferred because it is not easy to remove the sheet winding around the conveyor roller.
To address the problem, an image forming apparatus has been proposed that includes a cooling unit that cools a conveyed sheet at the downstream side of a fixing device in the sheet conveying direction to thereby reduce the curl at the opposite ends of the sheet in the sheet conveying direction (see Japanese Utility Model Application Laid-Open No. S62-146149).
The image forming apparatus described in Japanese Utility Model Application Laid-Open No. S62-146149 causes the cooling unit to cool the entire sheet and reduces evaporation of moisture, which can prevent the sheet from curling at the opposite ends in the sheet conveying direction to a certain extent but cannot sufficiently suppress curling at the opposite ends in the sheet conveying direction that may be caused by relative difference between shrinkages. Accordingly, the possibility of a winding jam still remains.
It is thus an object of the present invention to provide an image forming apparatus capable of preventing a winding jam of a sheet that can be caused by curling of the sheet at the front end in the sheet conveying direction.